Field of the Invention
The present invention relates to a particle filter of metal foil having mutually adjacent channels through which a fluid can flow. Each channel has at least one inlet and an outlet. The particle filter also has adjacent first and second channels. The first channel has an open entry cross section at a first end side of the particle filter. The invention also relates to a process for producing a particle filter.
European Patent 0 134 002 B1 has disclosed a diesel exhaust filter made from woven wire cloth and a process for its production. The diesel exhaust filter is constructed from layers which can be placed on top of one another or shaped helically to form an assembly. A layer includes a corrugated or folded screening cloth and a planar, continuous or perforated covering layer. Two end surfaces of the diesel exhaust filter are constructed in such a way that a closed end-face section lies opposite an open end-face section and an end-face section is closed by pinching. The corrugated or folded layer is pressed onto the planar layer in folds for that purpose.
It is accordingly an object of the invention to provide a particle filter of metal foil and a process for producing a particle filter, which overcome the hereinafore-mentioned disadvantages of the heretofore-known products and processes of this general type and which allow simplified production of the particle filter while at the same time producing a large surface area in the particle filter.
With the foregoing and other objects in view there is provided, in accordance with the invention, a particle filter, comprising first and second end sides. A metal foil forms walls defining mutually adjacent channels through which a fluid can flow. Each of the channels has at least one inlet and at least one outlet. The channels include first and second adjacent channels defined by the walls formed of the metal foil. The first channel has an open entry cross section at the first end side extended at least partway into the first channel. The second channel has an open exit cross section at least substantially corresponding to the entry cross section. At least one of the walls defining the first channel has perforations formed therein as filter passages leading to the second channel. A closure is disposed at the first channel, opposite the entry cross section and toward the second end side, for closing off the first channel to the fluid, at least as far as possible.
The fact that the walls of the first and second channels are made from metal foil means that they each have a large surface area which comes into contact with the fluid. While only the individual filaments of the cloth are available as surface area when using a woven wire cloth, a wall which helps to form the first channel has a surface that is continuous apart from the perforations. The perforations, acting as filter passages leading to the adjacent second channel, also have a surface with which the fluid can come into contact. Therefore, compared to a woven wire cloth, a perforated wall of that type has a larger surface area which also has a larger active surface area, for example, either when provided with a suitable coating or when the material of the metal foil is selected appropriately. It can be utilized for catalytic or other reactions or possible applications of a particle filter of that type.
The advantage of a large surface area is combined with the advantage that such a particle filter can be produced in a small number of working steps. The perforations which are required are, for example, prefabricated in the metal foil. The corresponding shaping to form the individual channels is advantageously completed in a single working step, irrespective of whether or not the metal foil has perforations. By way of example, it is advantageous if all of the walls which form the channels have perforations, so that during production the metal foil or foils can be processed independently of position and orientation.
Furthermore, perforating metal foil enables an accurate position of the filter passages to be achieved in the subsequent particle filter. While woven wire cloth is at risk of filaments shifting during processing, that is impossible in the case of perforations in the metal foil. That type of filter passages also enables the density of perforations to be varied over the metal foil and therefore the channel wall which is to be formed. It also enables a diameter of such a perforation to be varied. That option can be employed in particular if different filter stages are to be formed in the particle filter.
In order to avoid a high pressure loss across the particle filter, the second channel has an open exit cross section which corresponds to the entry cross section. As a result, it is possible for the pressure loss to be set approximately proportionally to the number and dimensions of the perforations. In accordance with another feature of the invention, the closure of the first channel of the particle filter is constructed in such a way that it does not allow any fluid to pass through. In this case, the filter passages form the only entry to the second channel. The closure of the first channel serves as a barrier wall, so that the fluid is forced through the filter passages. Particles contained in the flow of fluid which accumulate at the filter passages are then collected in the region of the closure. This can be assisted, for example, by providing a type of cage in the region of the closure. Due to the formation of the flow in the region of the closure, it is possible for a dammed region of the fluid formed at that location to be utilized in such a way that although particles do reach that region, they are then deposited in that region. Consequently, the filter passages remain clear and the particle filter requires fewer regeneration cycles. The particle filter may have suitable regeneration measures for regeneration such as, for example electrical heating, a catalytic coating or the like, in the region of the closure.
In accordance with a further feature of the invention, in order to simplify production of the particle filter, the first channel and the second channel have the same structure but are disposed in opposite directions relative to one another. This means that only one production tool is required for a metal foil. In the case of a particle filter having a layered structure, all of the metal foils can initially pass through production in one direction, and are only subsequently turned so that they alternate in opposite directions relative to one another. Advantageously, the first and second channels also form a honeycomb body which can preferably be produced in this way, with first and second channels alternating.
In accordance with an added feature of the invention, the walls of the first and second channels are formed from a single metal foil. This enables the metal foil to be unwound from a roll of metal foil, then subjected to a desired perforation step and for a desired shape to be imposed on the metal foil in the processing station which follows. The metal foil can then be formed into the particle filter either in wound or layered form. It is only at this working step that it is necessary for the metal foil to be cut off the roll of metal foil. The particle filter which has been layered or wound in this manner has joins that are produced, for example, by brazing, at locations where the individual walls touch one another.
In accordance with an additional feature of the invention, it is preferable to use a metal foil which has a coating before it is processed. This coating may either be of a catalytic nature, with the result that the surface of the particle filter is once again increased in size considerably due to the coating, or the coating may also include a joining element, such as, for example, brazing material, for joining walls of the particle filter which are in contact with one another. For this purpose, by way of example, the joining element is applied to the metal foil in strip form during or before the processing to form the particle filter. The joining element can also be applied, for example, to a suitable coating of the metal foil.
In accordance with yet another feature of the invention, in order to increase the surface area of the particle filter, it has also proven advantageous if the first and/or second channel has a tapering cross section. This cross section is preferably in the shape of a wedge. In the case of the first channel, the tapering cross section serves as an inlet and, as a result, reduces the pressure loss of the fluid flowing in. Furthermore, the active surface area which is acted upon by fluid is increased in size, since the fluid flows onto the surface at an angle. At the same time, this configuration enables particles which have accumulated at a filter passage to be, as it were, washed off by the fluid flowing onto the particles. As a result, the particles which are to be filtered out are moved onward into the region of the closure of the first channel. This movement of the particles is assisted by the fact that opposite walls of the metal filter in each case have perforations. As a result, a laminar flow is formed along these walls, with the fluid remaining in motion along this laminar flow. Due to turbulence formed in the central region of a channel of this type, the particles are carried onward, over the length of the channel, into the region of the channel closure, where they can be deposited.
A further important parameter of a particle filter is the pressure loss which it causes. However, to enable a large surface area to be formed in combination with a high filter action but without a high pressure loss, tests have shown that it is expedient to adapt the diameter of the filter passages. Therefore, in accordance with yet a further feature of the invention, the filter passage is a hole in the metal foil with a size of between 3 and 25 xcexcm, preferably 5 xcexcm. It is possible to optimize these otherwise contradictory parameters with a diameter of this type. This is assisted if the particle filter has approximately between 80,000 and 120,000 filter passages per m2 of wall. In this case, the square meter of wall is defined in such a way that the flowing fluid can flow onto it.
Since the particle filter, particularly when used in motor vehicles, is exposed to high temperatures, it is necessary for it to be thermally stable and also stable with respect to mechanical vibrations. In accordance with yet an added feature of the invention, this can be achieved with a metal foil which makes it possible to produce a wall with filter passages having a wall thickness between 20 xcexcm and 65 xcexcm, preferably between 30 xcexcm and 40 xcexcm. Particularly if the wall is between 30 xcexcm and 40 xcexcm, it is possible to produce the channels without major outlay when producing a particularly lightweight particle filter which, nevertheless, has sufficient stability and strength during operation.
In accordance with yet an additional feature of the invention, the coating of the particle filter is applied after the channels have been produced.
With the objects of the invention in view, there is also provided a process for producing a particle filter, especially the particle filter described above, from metal foil, which comprises pulling the metal foil out of at least one endless storage device, applying a joining element, in particular in strip form, to the metal foil, shaping the metal foil for producing subsequent channels and winding or stacking the metal foil to form first channels and second channels oriented in opposite directions. The first channel has an open entry cross section disposed at a first end side of the particle filter and extending at least partway into the first channel. The first channel also has a closure disposed opposite the entry cross section toward a second end side of the particle filter. Contact surfaces of the channels bearing against one another are permanently joined, to create the particle filter exclusively from metal foil.
In accordance with another mode of the invention, the metal foil is coated before or after the above steps.
In accordance with a concomitant mode of the invention, the metal foil is perforated before or after the above steps.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a particle filter of metal foil and a process for producing a particle filter, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.